As shown in FIG. 1, in an operational transconductance amplifier 100, a current source 110 provides a bias current I1 to a differential pair 108 of which NMOS transistors M1 and M2 are controlled by a pair of differential input voltages V− and V+ to produce two currents I2 and I3 respectively, a current mirror 102 composed of two PMOS transistors M3 and M4 mirrors the current I2 to produce a current I4, a current mirror 104 composed of two PMOS transistors M5 and M6 mirrors the current I3 to produce a current I5, a current mirror 106 composed of two NMOS transistors M7 and M8 mirrors the current I4 to produce a current I6, and the output current Io of the operational transconductance amplifier 100 is produced from the difference between the currents I5 and I6. The gain of the operational transconductance amplifier 100 is
                    GM        =                                            ⅆ              Io                                      ⅆ                              (                                                      V                    +                                    -                                      V                    -                                                  )                                              .                                    [                  Eq          ⁢                      -                    ⁢                                          ⁢          1                ]            Ideally, the gain GM of the OTA 100 should be constant. However, in reality, the output current Io tends to be saturated with the increase of the difference between the input voltages V+ and V− of the OTA 100, as shown in FIG. 2, that is to say, the gain GM of the OTA 100 will be less and less and go into slew rate limit range when the difference between the input voltages V+ and V− becomes larger. It is because that the bias current I1 provided by the current source 110 of the OTA 100 is constant, which causes that the differential current d(I2−I3) produced by a same differential voltage d(V+−V−) becomes less and less when the difference between the currents I2 and I3 reaches some certain value, and further causes that the variation dIo of the output current Io becomes less and less, so that the gain GM becomes less and less. For some cases, for example response to a load transient, a fast response speed is needed, whereas the OTA 100 cannot have such fast response speed due to the slew rate limit.
Therefore, an improved operational transconductance amplifier is desired.